Question

How do you evaluate `cos(-315)`?

Answer 1

Since all trig functions are periodic, we may add multiples of `360^0` (in the tangent function even multiples of `180^0`)

So `cos(-315^0)=cos(-315^0+360^0)=cos45^0`

An angle of `45^0` means an isosceles right-angled triangle with sides in the ratio `1-1-sqrt2`

The cosine of the angle will then be `1/sqrt2=1/2 sqrt2~~0.707`